Process control systems typically include various components for controlling various process parameters. For example, a fluid process control system may include a plurality of control valves for controlling flow rate, temperature, and/or pressure of a fluid flowing through the system. The end product is dependent on the accuracy of the control of these parameters, which is, in turn, dependent on the geometry and characteristics of the control valves. Control valves are, for example, specifically designed and selected to provide for particular flow capacities and pressure changes. When these characteristics are compromised, the quality of the end product may be affected.
In some applications (e.g., combustion turbine applications), it may be necessary to provide a control valve that is operable in a choked flow condition. Choked flow occurs when the velocity of fluid flowing through the control valve reaches supersonic speed (e.g., about 1070 feet per second for fuel flowing through the control valve for gas turbine electricity generation applications).
FIGS. 1 and 2 illustrate one known example of a control valve 100 that is operable in a choked flow condition. As illustrated, the control valve 100 includes a valve body 104 and a trim assembly 108 arranged in the valve body 104. The valve body 104 defines an inlet 112 and an outlet 116 connected by an annular valve port 120. The trim assembly 108 includes a seat ring 124, a seat ring retainer 128, and a valve plug 132. The seat ring 124 is arranged in the valve port 120. The seat ring retainer 128 is seated against the seat ring 124 to retain the seat ring 124 in position within the valve port 120. The valve plug 132 movably interacts with the seat ring 124 to control fluid flow through the valve port 120 (and thus the control valve 100).
The known flow control valve 100 suffers from many problems, however. Because fluid flowing through the valve port 120 tends to take the path of least resistance (i.e., the easiest route), most of the fluid flowing through the valve port 120 tends to flow through the valve port 120 at or through a front portion 136 of the perimeter of the valve port 120 (closest to a front side 140 of the seat ring 124 and a front side of the valve plug), as opposed to the rear portion 144 of the valve port 120 (opposite the front portion 136). In other words, fluid is not evenly distributed to and around the perimeter of the valve port 120 and the valve plug 132 when flowing from the inlet 112 to the outlet 116 through the valve port 120. As a result of this uneven distribution, a swirling effect is created, whereby fluid flowing through the valve port 120 at the rear portion 144 of the valve port changes direction several times before actually entering and flowing through the valve port 120. This change in direction causes a reduction in flow velocity, which also reduces the pressure of the fluid relative to the pressure of the fluid flowing through the valve port 120 at the front portion 136, thereby inducing an unbalanced velocity profile and an unbalanced pressure profile across the known control valve 100.